Tontotralman (who points out that he's NOT a carpenter) built this one-person slide on truck camper to fit inside his 6' truck bed with the tailgate shut. There's no cabover, but he still managed to fit a raised sleeping area inside.
It's built on 2x2 framing with OSB panels, and 1-1/2 styrofoam insulation. The interior is oriented sideways, with a sideways 6.5 foot wide bedroom area over a storage compartment, and front entrance area with storage cupboards on the left and kitchen area on the right.
On the outside he has some storage space between the walls and the bed, and a small electrical hookup. I like the split door idea that allows easy access.
Part 2 shows the finished body with corner trim, ceiling vent, water storage, the completed kitchen area (which looks much like a home kitchenette).
Part 3 shows the aerodynamic nose attached as a cabover extension to be used for lightweight storage in the future. He also explains the split door that allows for an exit when the tailgate is up. A highlight of Part 3 is his upgraded version of a "redneck air conditioner" made from a fan and an ice cooler and upgraded with copper coil and water pump.

Here's a really awesome DIY Slide On Four Wheel Camper Style Pop-Up Truck Camper RV by 'notesfromavagabond.' It's built on an aluminum frame he welded together himself and includes a few innovative features, such as a fold out room and a simple roof-lift mechanism.
(Thinking about building your own truck camper? (You can build them out of wood, too!) If you haven't yet, check out my article on How To Build Your Own DIY Truck Camper RV)